On the outer surface not exposed of a pipe installed under the ground for transporting, for example, a fluid such as a gas or a liquid, a defective portion such as those listed below may be produced:
(1) An accumulation of foreign matters caused by the production of rust; and
(2) A thinner portion caused by corrosion and the like.
Presence of an accumulation of foreign matters as mentioned above caused by the production of rust on the outer surface of a pipe causes such problems as reduction of the pipe wall thickness because of an extreme brittleness of the accumulation and easy peeloff from the pipe, finally producing a hole in the portion of the accumulation of foreign matters of the pipe. Presence of a thinner portion as mentioned above on the outer surface of the pipe results also in such problems as production of a hole in the portion having the thinner portion of the pipe. It is necessary therefore to promptly detect the above-mentioned defective portion on the outer surface of the pipe, and replace the pipe having such a defective portion with new one.
As a method for detecting a defective portion on the outer surface not exposed of a pipe, a method using ultrasonic waves is known, which comprises: transmitting ultrasonic waves toward the inner surface of a pipe to be tested in the interior of the pipe, receiving reflected waves of the transmitted ultrasonic waves, measuring the time required up to receiving the reflected waves, and detecting a defective portion on the outer surface of the pipe by means of the time required up to receiving the reflected waves.
However, the above-mentioned method using ultrasonic waves has the following problems:
(a) An error is often contained in the result of detection of a defective portion, thus preventing accurate detection.
(b) The range of a single run of detection, being only a point, is very narrow. It thus requires much time and labor for the detecting operation, leading to a low operating efficiency.
Under such circumstances, there is a strong demand for the development of a method and an apparatus which permit certain, easy and efficient detection of an accumulation of foreign matters or a thinner portion as a defective portion on the outer surface not exposed of a pipe, but a method and an apparatus provided with such properties have not as yet been proposed.